Retroreflective materials can return light that enters the material in the direction of the light emitting source. Retroreflective sheets are used in a wide range of applications for traffic safety, personal safety, and the like. Retroreflective sheets are generally used in various articles such as road signs, barricades, license plates, road surface markings, and marking tape, as well as in retroreflective tape for vehicles and clothing.
Two types of retroreflective sheet are known, namely cube corner sheets and microsphere-based sheets. Microsphere-based sheets (also referred to as “bead” sheets”) generally retroreflect incoming light rays by using a combination of a plurality of microspheres at least partially embedded in a binder layer, and a mirror reflective or diffuse reflective material (for example, pigment particles, metal flakes, vapor deposition coating, and the like). Cube corner sheets (also referred to as “prism shaped” sheets) generally have a thin transparent layer with a first surface that is essentially flat, and a second structured surface with a plurality of geometric structures, and all or a portion of the structures have three reflective surfaces to form cube corner elements.
In recent years, there have been proposals to use a retroreflective sheet on license plates (also referred to as number plates. Same hereinafter in this disclosure) in order to enhance the visibility of the license plate. On the other hand, automatic license plate reader (ALPR) systems that are installed on roads or proximal thereto are conventionally widely used. ALPR systems typically detect and recognize a vehicle using a camera system. Examples where ALPR systems are used include execution of road and traffic laws, investigations of vehicles involved in crimes, and access control to facilities. An ALPR system typically uses an infrared camera in order to read the number plate, and therefore infrared light is irradiated by the number plate. However, there are cases where the number plate is difficult to read because of halation caused by retroreflecting the infrared light. Therefore, a retroreflective sheet has been proposed that includes an infrared light absorbing agent in order to suppress the halation.
Patent document 1 (Japanese PCT application 2013-508749) discloses “a retroreflective sheet containing a cube corner element, a reflective body layer adjacent to the cube corner element, and an infrared impermeable material”.
Patent document 2 (Japanese Unexamined Patent Application 2014-024537) discloses “a sheet for number plate, containing a cesium tungsten oxide, and a substrate”.